This invention relates to a method for treating bitumen froth produced from tar sand by a hot water extraction process plant. More particularly, it relates to a system for pumping froth, diluted with light hydrocarbon, from a scroll-type centrifugal separator to a disc-type centrifugal separator within the two-stage centrifuge circuit that is conventionally used to recover the bitumen from the froth.
One of the world's largest reservoirs of hydrocarbons is the Athabasca tar sand deposit in Northern Alberta. The oil or bitumen from this deposit is presently being extracted using the known hot water process.
In general terms, this process involves mixing tar sand with water and steam in a rotating tumbler to initially separate the bitumen from the water and solids of the tar sand and to produce a slurry. The slurry is diluted with additional water as it leaves the tumbler and is introduced into a cylindrical primary settler vessel having a conical bottom. The largest part of the coarse sand particles settles out in the vessel and is removed as an underflow and discarded. Most of the bitumen and minor amounts of solids and water form a froth on the surface of the vessel contents. This froth overflows the vessel wall and is received in a launder extending around its rim. It is referred to as primary froth. A middlings stream, comprising water, fine solids (-325 mesh), and a minor amount of buoyant and non-buoyant bitumen, is withdrawn from the mid-section of the vessel and is pumped to a sub-aeration flotation cell. Here the middlings are agitated and aerated to an extent greater than that within the primary vessel. The middlings bitumen and some water and solids become attached to the air bubbles and rise through the cell contents to form a froth. This froth, referred to as secondary froth, is recovered in a launder and may then preferably be settled to reduce its water and solids content. The primary froth and settled secondary froth are combined and preferably deaerated and heated with steam in a column. Typically the deaerated froth comprises 62% bitumen, 29% water and 9% solids. The temperature of the froth after deaeration is typically 185.degree. F.
Following deaeration, the froth is pumped through a feed conduit to a two-stage dilution centrifuging circuit. In the first step of this circuit, a hydrocarbon diluent is injected into the feed conduit to mix with the froth. The diluent, usually naphtha, is added to reduce the viscosity and specific gravity of the froth bitumen phase and render it amenable to centrifugal separation. The diluted froth is then treated in one of a battery of scroll separators. This separator battery removes most of the coarse particles from the froth being treated. The scroll product is then pumped through one of a battery of disc separators to remove the remaining fine solids and water and produce a relatively clean, diluted bitumen stream.
It is known that emulsification of the bitumen, solids and water takes place as the froth moves through the process. This emulsification affects the quality of the bitumen product obtained from the disc separators. That is, the water and solids content of the disc product increases due to upstream emulsification.
In order to obtain a disc product which is acceptable for utilization in downstream bitumen upgrading units, it is conventional to add a chemical demulsifier to the feed stream just before it enters the disc separator. When one considers the size and throughput of a commercial hot water extraction plant, it will be appreciated that the cost for such demulsifier addition is substantial.
In accordance with this invention, it has been discovered that the problematic emulsification of the froth components occurs after the hydrocarbon diluent has been added. More particularly, as a result of work carried out in a test circuit, it has been found that if the deaerated froth is rigorously agitated in a mixing tank prior to the addition of naphtha, and if a low shear progressive cavity pump is used to transfer the product from the scroll separator to the disc separator, then the water and solids content in the disc separator product is relatively low, i.e. in the order of 5% of volume or less. However, when a commercial-type high shear centrifugal pump is substituted for the progressive cavity pump in this circuit, the water and solids content of the disc separator product increases substantially and is higher than the 5-7% content deemed to be necessary for the downstream refinery-type upgrading units.